Fundamental autotag.pl functions

There were a few things in autotag.pl that I put in separate
functions. First of all, contains_word_char() is a function that makes
the decision whether some text contains a word (\w in Perl) character.
It works correctly with undefined values as well, whereas with warnings
turned on, a regular expression match on an undefined value will print a
warning. It's primarily useful because it doesn't show a warning; in order
to achieve that effect without a function you'd have to check whether the
string is defined every time.

The only slightly unusual function is read_yes_no(), which can be
given a Y or 1 default parameter to make the default true, and any
other parameter to make the default false. Thus, I can make the
read_yes_no() function accept different default values when the user
presses Enter or Space. In addition, the Backspace or Delete keys will
reverse the default. It's not flashy code, but it's very useful.